Exhaust gas sensors which exhibit voltage jumps upon change of the exhaust gases between reducing and oxidizing condition, customarily known as lambda sensors, are employed in various types of internal combustion engine systems to control the air-fuel ratio of the mixture being supplied to the internal combustion engine such that combustion will occur under optimum conditions and with a minimum of noxious exhaust gases. A control system of this type uses a comparator to determine if the output signal from the sensor is greater or less than a median voltage level (see German Patent Disclosure Document DE-OS No. 27 07 383 corresponding to U.S. Pat. No. 4,208,993, PETER, June 24, 1980). This intermediate or median voltage level forms a reference or control point, and which is set to be within the voltage jump of the output signal of the lambda sensor at the transition between oxidizing and reducing state of the exhaust gases, that is, at an air-fuel ratio at lambda=.lambda..sub.o. The control device which determines whether the mixture supplied to the engine should be changed in lean or rich direction responds to the output value from the comparator.
The actual voltage output levels which are compared with the set point provides an indication if the lambda sensor functions under operative conditions. The temperature at which operating condition is sensed is the same whether the supply of the air-fuel ratio is in the rich or in the lean range.
It has been found that such an arrangement is subject to repetitive connection and disconnection of the control system if the lambda sensor is not yet at its appropriate operating temperature. Prompt function of the air-fuel control system is, however, desirable. If the internal combustion engine, immediately after starting and as it is warming up, is operated under idling condition, the control system may be connected and disconnected repeatedly. This results in stumbling operation of the engine, with erratic recovery and, in general, uneven engine operation.